1. Field of the Invention
The present invention relates to information storage and to information storage on magnetic tapes and in particular to a magnetic tape cartridge and a drive therefor.
2. Description of the Prior Art
The need for storing all types of information in a digital form is increasing very rapidly. Such information can be any kind of computer based/generated data, data generated by document scanners, loggers, various forms of video or audio information, or a combination of any of these.
Tape is by far the cheapest high capacity media available today, and the suppliers of tape drives are constantly bringing out new drives with better performance (higher capacity, shorter access time, higher data rates, etc.).
Originally, tape drives were based upon the open-reel principle: The tape to be recorded was supplied on one reel which was mounted on the tape drive by the operator. The tape was then guided from the supply reel across the recording head to an empty take-up reel. As the tape drive was running, more and more tape was moved from the supply reel to the take-up reel. At the end, the tape was rewound from the take-up reel back on to the supply reel. Then the operator could remove the supply reel and replace it with another one.
This system is still in use in some data systems today. The requirement for a trained operator, the size of the supply reel (normally 10 inches or more in diameter), and no protection of the tape when the supply reel is removed from the tape drive, make such open reel systems less attractive today.
There are many tape cassette/cartridges in use today; however, they can typically be categorized into three basic groups: single reel cassette, dual reel cassette, and belt driven cassette or cartridge.
The single reel cassette is really a refined version of the original open-reel system. Well known examples of the single reel cassette, are the IBM 3480 (later enhanced into 3490 and newer models) and the DLT cassette (previously known as the Compact cassette). These cassettes are characterized by a square (normally quadratic or close to quadratic) housing containing a single tape hub (with tape). During operation, the cassette is inserted in the drive and one end of the tape is automatically pulled out of the cassette and onto a take-up reel inside the drive. The operation therefore resembles the old open reel system, except that the tape cassette is physically smaller, and designed so that loading and tape extraction can be done without operator involvement. These cassettes have an opening on one side of the housing to allow the tape end to be pulled out during insertion into the tape drive. At the bottom of the cassette house is an opening which allows a drive motor built into the tape drive to engage to the tape reel in the cassette.
These single reel cassettes have increased in popularity during the last few 10 years, because they offer relatively large tape length in a fairly small cassette housing. Also, the cost of building tape drives which can handle such cassettes effectively and reliably has decreased considerably.
Nevertheless, single reel tape cassettes have some basic drawbacks The most important one is that the tape always must be pulled out of the cassette onto the take-up reel inside the drive. This requires a fairly sophisticated mechanism, and it takes time. Many single reel cassette systems need between 20 and 60 seconds or more just to load the tape properly. This will significantly reduce the effective speed of the system (longer data access time).
Furthermore, if power is lost during operation, it is normally impossible to eject the cassette from the tape drive. For some applications and systems, this may be a severe drawback.
The dual reel cassette is well known in many different versions: 4 mm DAT or DDS cassette, 8 mm videocassette, the Philips audio (Compact) cassette and the VHS video cassette just to name a few.
The principle of the dual reel cassette is that both tape reels are located within the cassette housing. The tape moves from one reel to the other during operation. A portion of the tape housing is typically designed to be opened when the cassette is inserted into the tape drive, thereby exposing the tape itself. The read/write head of the drive, tape capstan spindle(s) and drive tape guides can then get into contact with the tape. In some cases, like the VHS, the 8 mm and the 4 mm cassettes, a section of the tape is actually pulled out of the cassette during insertion into the drive and is wrapped around the recording heads and over the drive tape guides.
Compared to the single reel cassette design, the dual reel cassette normally allows a fairly simple drive mechanism to be used, especially if the tape does not need to be partly pulled out of the cassette. Dual reel cassettes have two openings at the bottom where tape drive motors can engage the tape reels inside the cassette.
A drawback with dual reel cassettes is that there must be room for more than two full tape reels inside the housing. First of all, the distance from each hub to the inside of the cassette housing must be at a minimum equal to the maximum radius of a hub fully loaded with tape (plus a small tolerance/safety figure). Secondly, the distance between the hubs is a function of the diameter of a hub fully loaded with tape. It can be shown that the maximum distance required between the two hubs occurs when both hubs contain equal amount of tape (=half the tape length).
By definition, within the same form factor, the dual reel cassette thus has a capacity for less tape medium than a similar single reel cassette with the same form factor.
During the last years, the dual reel cassette has gained a lot of popularity, especially because the mechanical design of the tape drive can be made far simpler and more reliable than for drives utilizing single reel cassettes. The fact that the available total length within a given form factor is typically 60 to 70% less than for a comparable single reel cassette, however, has hampered its acceptance in high end professional systems focusing on maximum capacity.
The belt driven cassette is a variant of the dual reel cassette. The most widely used version is the QIC (quarter inch cartridge) and its smaller companion, the Travan cartridge. In this type of cassette a belt engages the tape by friction. When the drive motor moves the belt this will move the tape.
Compared to a dual reel cassette, the belt driven QIC cartridge is designed so that only one capstan motor is necessary for driving the cartridge reels at one point. That can simplify the drive construction even further. Like the dual reel cassette, however, the belt driven QIC cartridge has the same limitations with respect to available tape length as the dual reel cassette. Actually, because the belt driven cartridge typically needs to have some additional components (belt guides etc.) inside its housing, the available tape length may be less than for a similar dual reel cassette design.
All these alternatives have a transfer rate which is limited, among other features, by the drive system's rotation velocity. The time to access specific data is limited as well in the known alternatives by the fact that segments of the tape can only be read sequentially (one after the other) from one end of the tape to the other.
These problems do not limit the use of magnetic tape for data storage in relation to backup systems, but they gain importance in relation to library systems where information must be recorded and accessed at high rates, and in relation to video systems.